Reflection on refraction in multifocal intraocular lenses - Chapter 1: Introduction

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(1)CHAPTER 1 Introduction.

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(3) Introduction. INTRODUCTION Refraction of light is the phenomenon of change in the direction of light rays from one medium to another. In the eye the cornea and crystalline lens determine the refraction. Also, the crystalline lens has the ability to accommodate, which is the process in which the lens changes its refractive power, thus enabling people to read without glasses. In an emmetropic eye light rays focus perfectly on the retina, whereas in ametropia the focal point is outside the retina. Ametropia can be divided into three groups. Myopia, or near-sightedness, is the situation, where light rays come together into a focal point located in front of the retina. Hyperopia, or far-sightedness, is the condition where light rays focus behind the retina. In astigmatism light rays are bent differently through different meridians, leading to several focal points, causing an imperfect image. In presbyopia accommodation recedes with ageing. Laser vision correction In the 1980s corneal laser vision correction was introduced to treat refractive errors and give people independence from spectacles. Seiler did the first excimer laser treatment on a blind eye in 19851 and shortly afterwards MacDonald and colleagues performed excimer laser surgery on a seeing eye.2 In time, different surgical techniques were developed, which can be roughly divided into lamellar ablation and surface ablation. Laser in situ keratomileusis (LASIK) is a lamellar technique in which a corneal flap is created, either with a microkeratome or with a femtosecond laser, and stromal treatment is done with an excimer laser. After treatment the flap is repositioned, and patients have little post-operative pain and quick visual recovery. Surface ablation includes photorefractive keratectomy (PRK) and laser epithelial keratomileusis (LASEK). In surface ablation the corneal epithelium is removed, with a blade or brush, sometimes after the use of 20% alcohol to loosen the epithelium, and the corneal stroma is treated with an excimer laser, through the Bowman’s layer. In PRK an epithelial defect is present after treatment. In LASEK the epithelial layer is repositioned over the stromal bed after ablation. Surface ablation gives more postoperative pain and a higher incidence of post-operative stromal inflammation with corneal haze, but has the advantage over LASIK that it does not create a corneal flap as a result of which there is less chance of post-laser ectasia and other flap-related complications. Long-term results in predictability and visual acuity are comparable for both the lamellar and surface ablation techniques.3 A newer technique is small-incision lenticule extraction (SMILE) in which an intracorneal inlay is created with a femtosecond laser and extracted through a small incision, also made by the femtosecond laser. The proposed advantages are less induced dry eye, flapless surgery and minimal discomfort and inflammation. The. 9. 1.

(4) Chapter 1. disadvantage is that the technique is technically more challenging with a steep learning curve, which could lead to complications such as incomplete lenticule extraction and interface irregularities, with inferior visual outcomes.3, 4 Different treatments are possible with an excimer laser. In a myopic treatment the treatment profile is deepest centrally, resulting in flattening of the cornea, while in a hyperopic treatment the cornea is steepened by maximum treatment of the outskirts of the treated zone. Treatment of astigmatism induces the ellipsoid-shaped cornea to change into a spherical-shaped cornea (Figure 1). With a wavefront-guided laser not only spherical and astigmatic components (lower-order aberrations) can be treated, but also higher-order aberrations (small, complex refractive errors that are beyond defocus corrected by spectacles).. Figure 1. A. Myopic eye: after refractive laser surgery the central cornea is flattened. B. Hyperopic eye: after refractive laser surgery the central cornea is steepened. C. Astigmatic eye: after refractive laser surgery the ellipsoid-shaped cornea is changed into a spherical shape by flattening a specific axis (Reprinted with permission from Kim et al. Refractive surgery. Lancet 2019;393:2085-98.).. 10.

(5) Introduction. Cataract extraction / refractive lens exchange Cataract is the medical condition where the crystalline lens has become opaque, resulting in blurry vision or visual disturbance such as glare and halos. Cataract can be treated with surgical cataract extraction. The first forms of cataract extraction were documented in ancient India and were done by the so-called “couching” method, a technique in which the opaque lens is dislocated into the vitreous cavity with a sharp object.5 In time cataract surgery evolved from extra- and intracapsular lens extraction into modern phacoemulsification cataract surgery, which is the most frequent form of surgery in the Netherlands, with approximately 190,000 surgeries performed annually.6 In phacoemulsification the crystalline lens is emulsified with ultrasonic energy and aspirated from the eye, followed by implantation of an artificial intraocular lens (IOL) into the capsular bag. In refractive lens exchange (RLE) or clear lens extraction, phacoemulsification is performed before cataract has developed. In the case of RLE not cataract but treatment of the refractive error is the indication for surgery. Multifocal intraocular lenses Monofocal IOLs treat spherical refractive errors (myopic or hyperopic) and provide clear vision at the focal point of the IOL implanted, either near or distance. In the case of corneal astigmatism, toric IOLs can be chosen to achieve emmetropia and thus clear vision. When patients opt for greater spectacle independence one can offer them a multifocal IOL. In a bifocal IOL light is focused from near and distance objects at the same time. In trifocal IOLs intermediate objects are also in focus. Multifocal IOLs come in different optical designs: refractive, diffractive or a combination of these designs. Refractive IOL models provide different annular zones of different refractive powers: one zone provides distance vision, and the other provides near vision. The performance of refractive lenses is pupil dependent, as the relative portions of the distance and near power vary among different pupil sizes. In diffractive IOLs multiple diffractive microstructures are present on the surface of the IOL in concentric zones. These rings lead to diffraction of light with two different foci. Diffractive IOLs are independent of pupil size. The division of light into several foci results in a reduction of contrast, and, because of the optical design, visual symptoms such as halo and glare are commonly reported side effects of multifocal IOLs. A relatively newer type of IOL is the extended depth of focus (EDOF) IOL. EDOF IOLs aim to give an elongated focus of vision with reduced incidence of halos and glare. There are many types of multifocal IOL on the market, but in this thesis only patients with an Acrysof ReSTOR (Alcon Laboratories, Inc.) IOL are included. The IOL comes in different add powers. In this thesis the SN6AD1 (+3.0 D addition) and the SN6AD3 (+4.0 D addition) were used. The Acrysof ReSTOR (Alcon Laboratories, Inc) is an aspherical single-piece hydrophobic acrylic 11. 1.

(6) Chapter 1. IOL with a 6 mm optic and a central 3.6 mm optic zone. It has 9 (SN6AD1) or 12 (SN6AD3) concentric diffractive rings (Figure 2). The diffractive rings come with a reduction in step height and step width from the centre to the periphery, which is called apodization. This leads to the direction of light into two different foci. The outer region of the optic does not have a diffractive design so there all the light goes through the distance power, which makes the lens distance-dominant with a large pupil.. Figure 2. Acrysof ReSTOR (Alcon Laboratories, Inc.) multifocal IOL with diffractive apodized design.. Intraocular lens calculation after refractive laser surgery The correct IOL power for a specific eye depends on the curvature of the cornea, the axial length and the estimated lens position in the eye. After previous corneal laser surgery IOL power calculation is less accurate. There are three main reasons for this inaccuracy. The first reason is incorrect estimation of the corneal refractive power because most instruments take incorrect measurements of the anterior corneal curvature. The second reason is an error in the assumed effective index of refraction. The index of refraction is based on the relationship between the anterior and posterior corneal curvature, which is altered by the laser. The third reason is incorrect estimation of the effective lens position by many IOL power calculation formulas.7, 8 If standard IOL power prediction formulas are used in eyes after previous myopic laser surgery, an overestimation of the corneal power will lead to a hyperopic refractive error post-operatively. With previous hyperopic laser surgery the opposite will occur, with a myopic refractive outcome. ASCRS calculator In the last decades several IOL power calculation formulas have been developed for eyes after refractive laser surgery. Some formulas use historical refractive data, others calculate IOL power without those historical data. Comparison of the IOL power prediction of 12.

(7) Introduction. these various formulas is time consuming, but the American Society of Cataract and Refractive Surgery (ASCRS) has developed a freely available online calculator to assist surgeons to select the appropriate IOL for these patients (www.iolcalc.org). Surgeons can enter the kerato-refractive data of their patient in the online calculator, and an output is automatically generated with IOL power calculation of the various available formulas. The online calculator has three modules, one for eyes after myopic laser surgery, one for eyes after hyperopic laser surgery, and one for eyes after radial keratectomy.. 1. Outline of the thesis The primary aim of this thesis is to investigate the visual and refractive results of multifocal IOL implantation in eyes with previous laser refractive surgery for myopia and hyperopia, and to compare various IOL power calculation formulas in these eyes. The secondary aim is to define the limits of implantation and to define parameters leading to higher success rates. In Chapter 2 the visual and refractive results of multifocal IOL implantation in a cohort with previous refractive laser surgery for myopia is described. Chapter 3 covers the results of a cohort with multifocal IOL implantation after previous refractive laser surgery for hyperopia. Chapter 4 describes retreatments in eyes with multifocal IOLs after previous refractive laser surgery. Treatment modes and complications are discussed. In Chapter 5 the various formulas available on the ASCRS online calculator are compared for previously myopic eyes with multifocal IOL implantation. In Chapter 6 the various formulas available on the ASCRS online calculator are compared for eyes with multifocal IOL implantation that had previously been lasered for hyperopia. Chapter 7 is a discussion of the research findings.. 13.

(8) Chapter 1. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. . 14. Seiler T, Bende T, Wollensak J. [Correction of astigmatism with the Excimer laser]. Klin Monbl Augenheilkd. 1987;191(3):179-83. McDonald MB, Kaufman HE, Frantz JM, Shofner S, Salmeron B, Klyce SD. Excimer laser abla tion in a human eye. Case report. Arch Ophthalmol. 1989;107(5):641-2. Kim TI, Barrio J, Wilkins M, Cochener B, Ang M. Refractive surgery. Lancet. 2019;393(10185):2085-98. Krueger RR, Meister CS. A review of small incision lenticule extraction complications. Curr Opin Ophthalmol. 2018;29(4):292-8. Basic and Clinical Science Course: Lens and Cataract: section 11. San Francisco: American Academy of Ophthalmology; 2014-2015. Henry Y. Cataract kwaliteitsregistratie. Nederlands Oogheelkundig Genootschap 2018. Hoffer KJ. Intraocular lens power calculation after previous laser refractive surgery. J Cata ract Refract Surg. 2009;35(4):759-65. Koch DD, Wang L. Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg. 2003;29(11):2039-42..

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